There are conventionally known, as hydraulic transmissions, (i) pure hydraulic transmissions (e.g., hydrostatic transmissions (HST)) which convert all input power supplied from an engine into oil pressure and transmit it and (ii) hydro-mechanical (power-split type) transmissions (HMT) which hydraulically transmit part of input power while mechanically transmitting the remaining part. The latter transmissions (HMT) have the advantage of achieving higher efficiency than the former transmissions (HST) since they convert only part of mechanical power into hydraulic power and the transmission efficiency of mechanical power is high. For this reason, the hydro-mechanical transmissions are said to be ideal transmissions for vehicles subjected to significant load fluctuations such as bulldozers and wheel loaders and therefore some of them are, in fact, employed in such vehicles.
A typical hydro-mechanical transmission (HMT) attains variable speed characteristics by a planetary gear mechanism. Of the three elements (i.e., a sun gear, a carrier equipped with a planetary gear, and a ring gear) of the planetary gear mechanism, the first element and the second element are coupled to the input shaft and the output shaft respectively, while the third element is coupled to the hydraulic pump or hydraulic motor. The rotating speed of the hydraulic pump or hydraulic motor is varied, thereby changing the rotating speed of the output shaft.
The above HMT is classified into two types. One is known as “the output split type” in which the hydraulic pump or hydraulic motor coupled to the planetary gear mechanism is connected by a hydraulic circuit to another hydraulic pump or hydraulic motor which is in turn coupled to the input shaft of the transmission so as to have a constant speed ratio. The other is known as “the input split type” in which the hydraulic pump or hydraulic motor coupled to the planetary gear mechanism is connected by a hydraulic circuit to another hydraulic pump or hydraulic motor which is in turn coupled to the output shaft of the transmission so as to have a constant speed ratio. Further, the output-split type and input-split type are respectively classified into six types according to which of the three elements of the planetary gear mechanism is coupled to the hydraulic pump/motor, the input shaft or the output shaft so that 12 types are available in total as basic combinations.
One prior art technique associated with the invention is disclosed in Japanese Published Unexamined Patent Application No. 2001-200900. The transmission disclosed in this publication includes a hydraulic transmission and a mechanical transmission having a planetary gear mechanism. The hydraulic transmission is driven by the mechanical transmission so that they interact with each other, operating with high efficiency over a wide range of operating conditions.
Next, reference is made to FIG. 18 (a) to describe a conventional output-split type transmission (HMT) having two pump-motors (which serve as a pump and a motor). In the transmission 100, a first gear 103 is fixed to an input shaft 102 to which motive power from an engine 101 is input, and a second gear 104 in mesh with the first gear 103 is fixed to a shaft 105a of a first pump-motor 105. Fixed to an input shaft 102 is a sun gear 107 of a planetary gear mechanism 106. A plurality of planetary gears 108 mesh with the outer circumference of the sun gear 107. Each planetary gear 108 is borne by a planetary carrier 109 to which an output shaft 110 is fixed. A ring gear 111 meshes with the outer circumference of the planetary gear set 108. A third gear 112 meshes with the outer circumference of the ring gear 111 and is fixed to a shaft 113a of a second pump-motor 113. Herein, the first pump-motor 105 and the second pump-motor 113 are hydraulically connected to each other through a piping 114.
In such an arrangement, when the rotating speed of the second pump-motor 113, in other words, the rotating speed of the ring gear 111 is zero, the motive power transmitted by the medium of oil pressure is zero so that all motive power is transmitted through the mechanical mechanism.
On the basis of the rotating speed of the output shaft 110 at that time, the operation of this transmission will be described.
(1) When increasing the speed of the output shaft 110, the second pump-motor 113 receives motive power through the medium of oil pressure and is driven so as to increase the speed of the output shaft 110. At that time, the first pump-motor 105 acts as a pump while the second pump-motor 113 acting as a motor, so that energy flows from the first pump-motor 105 toward the second pump-motor 113 through the medium of oil pressure. At that time, the horse power transmitted by the hydraulic power becomes plus (+) as indicated by line A-B of FIG. 18(b), so that hydraulic power is transmitted, in a forward direction, i.e., from the input shaft 102 toward the planetary gear mechanism 106.
(2) When reducing the speed of the output shaft 110, the second pump-motor 113 receives motive power from the planetary gear mechanism 106, rotating in a direction opposite to the case (1). At that time, the second pump-motor 113 acts as a pump while the first pump-motor 105 acting as a motor, so that energy flows from the second pump-motor 113 toward the first pump-motor 105 through the medium of oil pressure. At that time, the horse power transmitted by the hydraulic power becomes minus (−) as indicated by line A-C of FIG. 18(b), so that hydraulic power is transmitted in a reverse direction, i.e., from the planetary gear mechanism 106 toward the input shaft 102.
In the input split type HMT (transmission 200) shown in FIG. 19(a), the planetary gear mechanism 106 is disposed on the side of the input shaft 102 whereas the first pump-motor 105 is on the side of the output shaft 110. In FIG. 19(a), those parts that are identical with or correspond to those of the transmission 100 shown in FIG. 18(a) are identified by the same reference numerals as of FIG. 18(a) and a detailed explanation of them is omitted.
The input split type transmission 200 operates as follows.
(1) When increasing the speed of the output shaft 110, the second pump-motor 113 acts as a motor while the first pump-motor 105 acting as a pump, so that energy flows from the first pump-motor 105 toward the second pump-motor 113 through the medium of oil pressure. At that time, the horse power transmitted by the hydraulic power becomes minus (−) as indicated by line A-D of FIG. 19(b), so that hydraulic power is transmitted in a reverse direction i.e., from the output shaft 110 toward the planetary gear mechanism 106.
(2) When reducing the speed of the output shaft 110, the second pump-motor 113 receives motive power from the planetary gear mechanism 106, rotating in a direction opposite to the case (1). At that time, the second pump-motor 113 acts as a pump while the first pump-motor 105 acting as a motor, so that energy flows from the second pump-motor 113 toward the first pump-motor 105 through medium of oil pressure. At that time, the horse power transmitted by the hydraulic power becomes plus (+) as indicated by line A-E of FIG. 19(b), so that hydraulic power is transmitted in a forward direction, i.e., from the planetary gear mechanism 106 toward the output shaft 110.
As such, in both the output split type and input split type transmissions, a forward energy flow and a reverse energy flow occur in the speed-up side and the speed-down side. The transmission efficiency of energy of this case will be examined below taking the output split type transmission 100 shown in FIG. 18 for example. Herein, the transmission efficiency of the mechanical unit is 95% and the transmission efficiency of the hydraulic unit is 80% (Generally, where a pump-motor is used, transmission efficiency is low). For easy comparison, a case where the output rotating speed of the hydraulic unit is increased by 0.5 to 1.5 when the output rotating speed of the mechanical unit is 1 is compared to a case where the output rotating speed of the hydraulic unit is reduced by 0.5 to 0.5 when the output rotating speed of the mechanical unit is 1.
FIG. 20(a) shows the case where hydraulic power flows in the forward direction. One-third (=0.5/1.5=0.333) the energy (1.0) output from the engine 101 flows to the hydraulic unit for increasing speed. Transmitted to the output shaft 110 are 0.633 (=0.667×0.95) part of energy from the mechanical unit and 0.267 (=0.333×0.8) part of energy from the hydrostatic unit. As a result, the overall efficiency becomes 0.9 (=0.633+0.267). The case where hydraulic power flows in the reverse direction is shown in FIG. 20 (b). Where the energy transmitted from the mechanical unit to the hydraulic unit for reducing speed is represented by E, the energy at the output side of the mechanical unit before splitting is 2E and the following equation is obtained.((1+0.8E)×0.95)=2E  (Equation 1)
From Equation 1, E=0.766 is obtained so that the overall efficiency is 0.766.
As just discussed, when hydraulic power flows in the reverse direction, a flow of large energy occurs in each element, causing poor efficiency. In other words, the forward flow of hydraulic energy is better than the reverse flow of hydraulic energy. As apparent from FIGS. 20(a) and 20(b), if part of energy is directed in the reverse direction, the energy that passes through the mechanical unit will increase. This entails a need for a larger planetary gear mechanism, which is disadvantageous in economical efficiency.
The previous technique relating to the invention, which is disclosed in Japanese Published Unexamined Patent Application No. 2001-200900, is designed to avoid the above-described situation in which energy flows in the reverse direction, by properly changing the transmission path which extends between the planetary gear mechanism and the output shaft. The technique disclosed in this publication, however, is complicated in the structure of the planetary gear mechanism and inevitably involves a multiplicity of gears which do not participate in energy transmission, increasing idling losses with the result that the transmission efficiency of the mechanical unit deteriorates. Furthermore, the technique disclosed in this publication has revealed such a drawback that since it is designed to shift gears by switching the transmission path between the planetary gear mechanism and the output shaft through engagement/disengagement of clutches, a so-called torque shortage (i.e., a momentary drop in the torque of the output shaft) or a gear change shock will occur if the timing of clutch engagement/disengagement is bad.
The invention is directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide a transmission having a very compact system configuration and capable of increasing energy efficiency over all speed regions from a low speed region to a high speed region, while providing improved operability free from a torque shortage.